This invention relates generally to electric discharge lamps and, more particularly, to flashtubes having a wall-stabilized arc discharge.
Flashtubes generally comprise two spaced apart electrodes within a sealed light-transmitting envelope having a rare gas fill, typically xenon, at subatmospheric pressure. Such lamps are connected across a large capacitor charged to a substantial potential, which is, however, insufficient to ionize the gas fill. Upon application of an additional pulse of sufficient voltage, the xenon is ionized and an electric arc is formed between the two electrodes, discharging the large capacitor through the flash tube, which emits a burst of intense light, usually of short duration. In many cases, a pulse voltage is applied between an external trigger wire wrapped around the envelope and the elctrodes; this is referred to as shunt triggering. Such external triggering may also be achieved by providing the exterior of the envelope with a conductive coating which is connected to a lead wire from the trigger pulse source by means of a narrow metal band slipped over the coated envelope. In other applications the lamp may be internally triggered by applying the pulse voltage directly across the electrodes, a technique referred to as injection triggering.
Typically, the envelopes of flashtubes are formed of quartz, borosilicate glass, or so-called ignition glass, all of which are relatively expensive, generally more difficult to draw and form, and pose special consideration with respect to providing strong metal-to-glass seals. This is particularly true of flashtubes having a wall-stabilized discharge, i.e., wherein the length of the arc path between electrodes is large in comparison with the inside diameter of the envelope. (See High Pressure Mercury Vapor Lamps and Their Applications, edited by W. Elenbaas, Philips Technical Ligrary, 1965, pages 3 and 242-243; also Light Sources, W. Elenbaas, published in U.S. by Crane, Russak and Company, Inc. New York, 1972, pages 127, 163 and 196). Soft glass has been mentioned for use as the bulb material of an electrode-stabilized flashtube, however such a lamp type has an inside diameter of the bulb which is large in comparison to the arc-length; for example see U.S. Pat. No. 3,766,421, column 4, lines 62-65, in which a T-12 bulb of lime glass having a diameter of 1.5 inches is used for a flashtube having an electrode spacing of about 0.100 inch and a loading of about 0.05 joules per flash. In the case of wall-stabilized flashtubes having loadings of several joules, however, it appears that only quartz and hard glasses, such as borosilicate and/or aluminosilicate, have been considered for use as the discharge envelope material, apparently in view of the anticipated thermal and acoustical shock effects upon the discharge confining wall.
In years past, the cost factor of hard glass or quartz flash tubes posed no particular concern in view of the relatively expensive power supplies employed. With the advent of integrated solid state circuitry, however, power supply costs have been reduced considerably, in fact, to the point that the flashtube cost is equivalent ot the once expensive circuitry. Consideration of the use of soft glass envelopes to reduce the cost of wall-stabilized flashtubes has been generally discounted with predictions of failure due to the high coefficient of thermal expansion and the suspected inability of soft glasses to withstand flashtube loadings of several joules.